The invention relates to a metal halide lamp provided with a discharge vessel with a ceramic wall which encloses a discharge space in which besides a rare gas also an ionizable filling comprising at least NaI is present, two electrodes having tips with a mutual distance EA being arranged in said discharge space which discharge vessel has an internal diameter Di over at least the electrode distance EA.
A lamp of the kind mentioned in the opening paragraph is known from EP-A-0 215 524. The known lamp, in which a high luminous efficacy goes hand in hand with excellent color properties (inter alia a general color rendering index R.sub.a .gtoreq.70 and a color temperature T.sub.c of between 2600 and 4000 K), is highly suitable as a light source for inter alia interior lighting. This lamp construction is based on the recognition that a good color rendering is possible when sodium halide is used as a filling ingredient of a lamp and a strong widening and inversion of the Na emission in the Na-D lines takes place during lamp operation. This requires a high coldest-spot temperature T.sub.kp in the discharge vessel of, for example, 1170 K (900.degree. C.). Inversion and widening of the Na-D lines imply that they take the shape of an emission band in the spectrum with two maxima at a mutual distance of .DELTA..lambda.. The requirement that T.sub.kp should have a high value excludes the use of quartz or quartz glass for the discharge vessel wall and renders the use of a ceramic material for the discharge vessel wall necessary.
The term "ceramic wall" in the present description and claims is understood to cover a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al.sub.2 O.sub.3 as well as metal nitride, for example AlN.
The known lamp combines a good color rendering with a comparatively wide range of the color temperature. The filling of the discharge vessel comprises at least Hg, Na halide and Tl halide. In addition, the discharge vessel preferably contains at least one element from the group formed by Sc, La, and the lanthanides Dy, Tm, Ho, and Er.
The known lamp has a lamp voltage during stable operation of between 70 and 110V, being the general accepted range for discharge lamps. In the known lamp this voltage is mainly sustained during stable operation by the mercury which forms part of the filling. However Hg forms a heavy burden on the environment in case it would be released, for instance at the end of the life of the lamp.